Scotland could end up voting for independence because those in favour of remaining part of the UK could leave it too late to make their voices heard, Alistair Carmichael has warned.

The Liberal Democrat Scottish Secretary admitted it was "not impossible" that the Yes campaign could triumph in September's referendum and called on Labour to play a greater role in the contest.

His intervention came as the pro-union campaign was rocked by an anonymous minister's claim - which has been denied at high levels in the UK government - that Scotland could join a currency union with the rest of the UK and keep the pound in the event of a Yes vote.

Considering the prospect of a vote for independence Mr Carmichael said: "It is not impossible. I am not expecting to lose, but it is eminently possible that they will be able to buy momentum with the advertising and campaign resource they have.

"If they do, it could all get very difficult."

Mr Carmichael said the "volume" of the No campaign needed to be increased. " The basic messages are the right ones. They are delivered in a fairly professional way. We just need to do more of it," he said.

Warning of the risk of complacency among unionists, he told The Observer: "The danger is that by the time they realise it could happen, it could be too late.

"Everybody needs to know that this is a serious contest, and one which it is not impossible that the nationalists could win."

He said the No campaign needed to match the nationalists' "hunger" for victory.

"We're never going to match them for the spend, but in terms of the hunger I think we have to match them for just how badly we want this," he said.

"That is always going to be a challenge, because for nationalists this is the issue that defines them, whereas for a Labour supporter, a Liberal or a Conservative, this can be an issue you care about but is not one that defines you.

"So that is where we need to work harder at motivating our people in a way that their people come ready motivated."

T he cross-party Better Together campaign is being led by Labour former chancellor Alistair Darling, but Mr Carmichael called for a wider range of voices from Ed Miliband's party to join the debate.

"You have in Alistair Darling a first-rate campaign head, but I would want to hear and would expect to hear a wider range of Labour voices coming into the debate as it gets closer to polling day," he said.

"The interventions of Gordon Brown have all been good and positive and helpful, and I'd like to see more of that."

Chancellor George Osborne and his Lib Dem deputy Danny Alexander were forced to intervene to insist that Scotland would not be able to form a currency union in the event of independence after an unnamed minister's comments appeared to undermine one of the key elements of the No campaign's strategy.

The two senior Treasury ministers said: "The Scottish Government are proposing to divorce the rest of the UK but want to keep the joint bank account and credit card.

"The UK would not put its taxpayers at risk of bailing out a foreign country and its banks. Parliament wouldn't pass it, and the people wouldn't accept it.

"Any suggestion to the contrary is wrong."

All three main parties at Westminster have officially ruled out sharing sterling with an independent Scotland and made it a key plank of the No campaign, but an unnamed minister was quoted in The Guardian as saying "of course" there would be an agreement on the pound, indicating that a deal could be done with Scotland in exchange for the UK's nuclear submarine fleet remaining at Faslane

"There would be a highly complex set of negotiations after a Yes vote with many moving pieces," the minister said.

"The UK wants to keep Trident nuclear weapons at Faslane and the Scottish Government wants a currency union - you can see the outlines of a deal."

The minister's comments were seized on by Deputy First Minister Nicola Sturgeon, who said they gave a big boost to the Yes campaign.